The present invention relates generally to ceramic matrix composites, and more particularly to ceramic matrix composites reinforced with inorganic fibers and wherein the matrix incorporates an addition of dispersed mica crystals.
The use of inorganic whiskers and fibers to reinforce glasses, glass-ceramics, and ceramics has long been practiced. The mechanism of strengthening of glass or ceramic bodies by fibers is considered to be that of load transfer by the matrix to the fibers through shear. This load transfer shifts stress from the glass or ceramic matrix to the relatively long, high modulus fibers, while the fibers at the same time may act to impede crack propagation in the matrix material. Whiskers are thought to impart strengthening by a similar mechanism, but load transfer to whiskers by the matrix is more limited due to the limited length and aspect ratio of the whiskers.
Among the fibers and whiskers which have been suggested for use as reinforcements for inorganic matrix materials are silicon carbide, silicon nitride, alumina and carbon whiskers. For example, U.S. Pat. No. 4,324,843 describes SiC fiber reinforced glass-ceramic composite bodies wherein the glass-ceramic matrix is of aluminosilicate composition. U.S. Pat. No. 4,464,475 describes similarly reinforced glass-ceramics comprising barium osumilite as the predominant crystal phase, while U.S. Pat. No. 4,464,192 describes whisker-reinforced glass-ceramic composites of aluminosilicate composition.
A principal objective of whisker reinforcement in glass, ceramic and glass-ceramic materials for high temperature applications is that of increasing the toughness of the material. A toughened ceramic material exhibits improved resistance to cracking failure from flaws sustained in use, offering the possibility of increased fatigue lifetime. As noted in U.S. Pat. No. 4,626,515, the addition of fiber reinforcement to glasses such as alkali-free alkaline earth aluminosilicate glasses can result in substantial strengthening, while whisker additions to those glasses were found effective to enhance the toughness of the glass. Composites comprising glass-ceramic matrix materials and incorporating both fiber reinforcement and whisker toughening agents (referred to as hybrid composites) are described in U.S. Pat. No. 4,651,987.
The addition of certain particulate materials to ceramic matrix composite systems, to serve as toughening agents for the matrix in preference to whisker additions, is described in U.S. Pat. No. 4,919,991. As noted in that patent, matrix additives such as silicon carbide particulates were found effective to improve properties such as transverse and interlaminar shear strength in glass-ceramic matrix composites comprising layered fiber reinforcement.
U.S. Pat. No. 4,935,387 describes the first use of mica materials as functional components of ceramic matrix composite structures. That patent shows that, in a fiber-reinforced ceramic composite structure, the presence of at least a thin layer of mica adjacent to the fibers in the composite imparts tough fracture behavior to the system. This effect was attributed to the ability of the mica to furnish a weak oxide fiber/matrix interface in the material which facilitated fiber pullout from the matrix. Good pullout performance was observed even at temperatures sufficiently high to produce fiber adhesion and/or embrittlement, and thus brittle composite fracture behavior, in similar composites not incorporating the mica.
One of fiber materials which has frequently been used for the reinforcement of ceramic matrix composites such as above described is silicon carbide fiber. Commercially available forms of this fiber, most commonly silicon oxycarbide fiber sold as Nicalon fiber, has been widely studied as a reinforcement fiber for glass-ceramic composites based on anorthite (calcium aluminosilicate) cordierite (magnesium aluminosilicate), spodumene (lithium aluminosilicate), and many other alkali and alkaline earth silicate systems.
Although silicon carbide fibers provide composites which are quite strong and tough at moderately elevated temperatures (eg., 600.degree. C.), reductions in failure stresses and strains to less than 50% of room temperature values can be observed at higher temperatures (eg., 1000.degree. C.). This strength decline is attributed to the oxidation of a weak graphitic interface layer, formed on the silicon oxycarbide fibers as an incident to the normal composite consolidation process. The fiber pullout characteristics of this layer, which are essential to the strength and toughness of these fiber composites, are lost through rapid layer oxidation at high temperatures resulting from of air penetration through microcracks in the ceramic matrix.
Several approaches to the solution of this problem are have been studied. Most recently, as noted in U.S. Pat. No. 4,935,387 above and in U.S. Pat. No. 4,948,758, tetrasilicic fluormica and other mica matrix and coating systems have been evaluated as a means of providing a weak oxide interface which should be more oxidation resistant than a carbon interface. However, although this approach provides composites showing fibrous fractures at 1000.degree. C., the strength of these composites is relatively low (40 to 50 Ksi at 25.degree. C. and 1000.degree. C.) due to low shear strength of the mica matrix which is controlled by the weak cleavage strength. Also, the need to use coated fibers adds complexity to the manufacturing process, and thus increases the cost of the products.
Accordingly, it is a principal object of the present invention to provide silicon-carbide-fiber-reinforced ceramic composites offering improved strength retention at elevated temperatures.
It is a further object of the invention to provide silicon carbide-reinforced ceramic composites offering improved resistance to oxidative embrittlement, and a method for making them which does not require the coating for the silicon carbide reinforcing fibers.
Other objects and advantages of the invention will become apparent from the following description thereof.